Synchronized direct current motor system



July 2, 1946. LEAR 2,403,097

SYNCHRONIZED DIRECT CURRENT MOTOR SYSTEM Filed Feb. 3, 1944 2Sheets-Sheet l INVENTOR. W/ZZM/ P. 154/? ATTORNEY July 2, 1946.

w. P. LEAR 2,403,097

SYNCHRONTZED DIRECT CURRENT MOTOR SYSTEM Filed Feb. 3, 1944 2Sheets-Sheet 2 777? [/72 fl ZZJLZTZQJ y if ,:|-|-|-f-|# IN V EN TOR.

ATTORNEY Patented July 2, 1946 r 2,403,097 srncnaomzsn n'nmc'r CURRENTMOTOR SYST William P. Lear, North Hollywood, Calif., assign or, by mesneassignments, to Lear, Incorporated, Grand Rapids, Michi, a corporationof Illinois Application February 3, 1944, Serial No. 520,894

- Claims. 1

Thisinvention relates to synchronized direct current motor drivesystems, and more particularly to such systems embodying anti-disparitycontrols for the motor drives.

The system of invention is particularly applicable to synchronouslycontrolling the operation of groups of accessories on aircraft.v Thelarger airplanes are provided with accessories, such as wing flaps,landing gear andso forth, that are operated in pairs, wherein each pairmust be moved in synchronism and maintained in aligned relation, inorder to provide efiective control of the airplane. Where direct currentmotors are used, because of their superior torque and speed controlcharacteristics, or because the available power source is directcurrent, the present invention provides a synchronizing control systemfor a plurality of such motor operators.

Various schemes have hitherto been proposed for synchronizing theoperation of two or more direct current motor drives, maintaining themin step with each other. However, the systems heretofore proposed havenot been efiective to correct any disparity between the positions of themotors or to eifect stoppage of the motors in the event that one or morethereof get out of synchronism with the others.

It is among the objects of this invention to provide a synchronizingarrangement for direct current motor drives, including anti-disparitycontrols; to provide such a system in which, if one motor drive isoverloaded or otherwise draws an excessive amount of current, all of themotor drives will be deenergized and stopped; to provide such a systemin which, if one motor drive reaches its limit of operation before theothers, it will be deenergized and stopped without affecting thecompletion of operation of the other motor drives; to provide such asystem embodying difierential relay means for correcting disparitybetween the several motor drives; and to provide such a system includinga remote positioning and contro1 arrangement operatively associatedtherewith.

These and other objects, advantages and features of the invention willbe apparent from the following description and accompanying drawings. Inthe drawings:

Fig. 1 is a schematic electric circuit arrangement for synchronizing theoperation of two direct current motor drives, including anti-disparitycontrol means.

Fig. 2 is a schematic electric circuit arrangement for synchronizing theoperation of two pairs of direct current motor drives includingantidisparlty control means.

of motors. posing relay coils are-connected to the source of Fig. '3 isa schematic electric circuit arrangement of a modified anti-disparitycontrol for the pairs of motor drives shown in Fig. 2.

Fig. 4 is a schematic electric circuit arrangement similar to Fig. 2,and including a remote position selecting and indicating controlmechamsm.

Generally speaking, in the arrangement of the present invention,synchronizing circuit connec tions are provided between two or moredirect current motors. Preferably, such synchronizing circuitconnections include one or more slip rings rotatable with thearmature ofeach direct current motor and connected to electrically equidistantpoints on the armature windings. Condoctors and brush means are providedto interconnect corresponding slip rings of the several motors. To guardagainst disparity between the motor positions, differential relay meansare provided. Such differential means preferably comprises one or moredifferential relays each having a pair of opposing coils, each of whichis connected in series with one motor or with one pair The oppositeterminals of the opelectric power through a back contact of the relay,which back contact is normally closed. A third coil is provided in eachrelay which is normally shunted by such back contact. Upon a substantialunbalance in the current requirements between each motor or between eachpair of motors, the difijerential relay will operate to open its backcontact. This deenergizes the opposing coils of the relay, disconnectingthe motors from the source of power, except for a small holding currentthrough the holding coil of the relay. Such holding coil maintains theback contact open until its holding circuit is interrupted by a suitablemeans.

The system is particularly applicable to the synchronous control of aplurality of' mechanical actuator units, such as for operating wingflaps or other accessories of an aircraft, for instance as illustratedin my copending application Serial No. 490,136, filed June 9, 1943, forMultiple actuator system, now Patent No. 2,366,734, issued January 9,1945. For simplification of presentation, only the reversible directcurrent electric motors forming part of such actuator units areillustrated in the drawings. Such motors are shown at In and 15 asincluding armatures H and I2 and field windings l3, l3 and H, H. Fieldwindings I3, I 3 or H, M are oppositely wound for selective reverseoperation of armatures II and I2 respectively. As disclosed in saidcopending application Serial No. 490,136 and in my copending 3application Serial No. 483.515, iiled April 17,1943, ior Unitarymechanical actuator device," both assigned to the same assignee as thiscase, each oi the motors Ill and II is adapted to be connected to adriven system, such as individual i wing flaps, through a quick actingelectromagnetic clutch and brake unit oi the type described and claimedin my Patent No. 2,267,114, issued December 23, 1941, ior"Electromagnetic clutch," and suitable reduction gearing. As describedin said patent, the quick acting magnetic clutch and brake unit is alsoeiiective to cause substantially instantaneous stopping oi the drivensys- H as connected in series with armatures II and I2, respectively.

Synchronizing circuit connections are provided between motors i and II.As shown, such synchronizing circuit connections comprise slip rings isand 20 which are rotatable with the armatures II and II, respectively.In a manner not shown ior the purpose oi simpliiying the drawings, ringsl3 and 20 are connected to electrically equi-distant points of thewindings oi armatures II and I2 as will be understood by those skilledin the art. Brushes 2| and 22 operatively engage slip rings l3 and 20,respectively. The brushes are interconnected by conductors 23. Theoperation oi the synchronizing arrangement is as iollows. Upon adii'ierence in the relative phase relation oi armatures II and i2, 9.circulating current will flow through conductors 23 causing a slowingdown oi the faster motor with respect to the slower motor, or viceversa. This arrangement maintains motors Ill and It in synchronousrotation. However, under severe conditions, the synchronizing force maybe insufilcient and motors ill and It may get out of step to. destroythe synchronous relation oi the drives. To prevent this, ananti-disparity control arrangement is provided by the present invention.

As shown, one terminal oi each oi the direct current motors l0 and I3 isconnected to one terminal oi the source of power diagrammaticallyillustrated at 23 as comprising a battery. Motor armatures I and I2 areconnected in series with field windings l3, l3 and l4, M, respectively.Field windings, I3 are connected through limit switches 24 and 24 toopposite terminals 21 and 23 oi a reversing switch 30. Similarly, fleldwindings l4 are connected through limit switches II and 32 to oppositeterminals 33 and 34 oi a reversing switch 3!. Limit switches 24, 28, 3|and 32 may be mounted in the control boxes oi the linear actuator .unitillustrated in my above mentioned copending application Serial No.483,515 and, as therein described, may be operated through suitablereduction gearing connected to the actuator and motor oi such units.Desirably, reversing switches Ill and 35 are ganged together ior unitaryoperation, as indicated at 33.

The anti-disparity control ior the system of Fig. 1 comprises adiiierential relay 40 having opposing low resistance coils 31 and 23.One terminal oi coil 31 is connected in series circuit relation withswitch 33, and one terminal oi coil 32 is connected in series circuitrelation with switch II. The opposite terminals of coils 31 and gaged byan armature 42 forming part of relay 40. Armature 42 is connectedthrough a normally closed push button 43 to the opposite terminal ofsource of power 25. Relay 4U likewise includes a third, or holding coil44 having one terminal connected through a junction point 45 and pushbutton 43 to source 25. Coil 44 is oi higher resistance than coils 31,38. The opposite terminal is connected through junction point 44 inseries with coils 31 and 38. It will be noted that armature 42 normallyshunts holding coil 44.

The operation of the system of Fig. 1 is as follows. Under normaloperation, wherein both motors l0 and I! are drawing substantially equalcurrents, coils 31 and 38 of relay 40 will be substantially equallyenergized in opposite directions, and armature 42 will remain in contactwith contact 4|. Should motor III, for instance, draw a substantiallydifferent amount of current than motor l5, due to unbalanced load ordrive conditions, relay 40 will become unbalanced. Such unbalance willcause armature 42 to be drawn from engagement with contact 4|. The shuntcircuit is thus removed from coil 44, retaining the armature 42 in itsopen position. Motors i0 and it are thus disconnected from source ofpower 25. A small current continues to flow in series through coil 44and coils 31 and 38. The motors are thus deenergized as are theclutches, and disconnected from their driven members through theclutches operated by their coils I8 and I1.

Another 'possible condition is that the motors continue to drawsubstantially equal current, but that the one reaches its end limit ofoperation oi driving before the other. Should motor III, for instance,reach its limit before motor l5, its

associated limit switch 24 or 26 will be opened depending upon thedirection of its rotation. Opening of such limit switch will cut motorII from source of power 25. Normally, the resulting unbalanced conditionof relay 40 would likewise out motor l5 from the source of power.However, resistances 41, 43, 43 and 50 are connected in parallel withmotors Hi and I5. Resistances 41, 43, 49 and 50 are preferably madesubstantially equal in value to the running resistance oi motors III orl5. When any one of limit switches 24, 23., 3|, 32 is opened, it engagesa back contact 5|, 52, 33 or 54 which inserts one of the saidresistances in series with the corresponding opposing coil 31 or 38 ofrelay 40. Hence, it limit switch 24 opens, it engages contact 5| andplaces resistance 41 in series with opposing coil 31. As resistance 41is substantially equal in value to the running resistance of motor l5,no unbalance oi diiierential relay 40 will occur despite continuedoperation of the other motor l5. Motor I! will thus be permitted tooperate until it reaches its limit oi operation and opens its limitswitch 3| or 32, whereupon motor l5 also will be disconnected fromsource 25. The system may be reset, after all motors have beendisconnected from the source of power, by depression of pushbutton 43which breaks the holding circuit ior relay coil 44, permitting armature42 to drop back into engagement with contact 4|.

' As mentioned hereinabove, the described a!- rangement is particularlyvaluable for use aboard aircraft tor operating in synchronism variousaccessories in pairs, such as wing flaps, dual rudders, multiple landinggear and so forth. Such elements must be motivated in synchronism orstep without any substantial disparity between the members of eachgroup. If the invention 3| are connected to a contact 4| normallyenarrangement is used, for instance, to control the 5positionorapairotwingflapsthroughthemedium or linear actuator units asillustrated in my said Patent No. 2,366,734, any substantial disparitybetween the positions of the wing flaps will cause a, deenergization andinstantaneous s pping of both motors Ill and it before any amount ofdisparity can occur to cause faulty control of the airplan Through theelectromagnetic clutch and brake arrangement used with the presentinvention, and described in my above mentioned Patent No. 2,267,114,deenergization or the motors eflects instantaneous stopping o! thedriven elements connected thereto. Normally, an operating tolerance upto about 30% disparity is permissible for the-operation of wing flaps.be sensitive, or for disparity control well within the range orpracticable disparity between a pair of wing flaps.

Fig. 2 illustrates how the arrangement oi Fig. 1 may be used to controlthe operation of two pairs or direct current motor drives. Thearrangement of Fig. 2 might be used, for instance,

to control inboard" and outboard" pairs of wing flaps on an airplanewith one pair 01 motors controlling the position of the inboard" flapsand the other pair that of the outboard flaps. The arrangement isbasically the same as that of Fig. 1. The first pair of motors mayinclude motors!!! and I" connected in parallel with each other to theterminals 21 and 28 of a reversing switch -30. The second pair of motordrives may include motors-i and I5" connected in parallel to theterminals 33 and 34 of a reversing switch 35. As in Fig. 1, reversingswitches 30 and 35 may be ganged as at 36. Motors I0 and iii" areprovided with slip rings i8 and i8", and motors l5 and N" are providedwith slip rings 20' and 20". Synchronizing circuit connections 28 areprovided as in Fig. 1. With the exception 01 the diilerences hereindescribed, the system shown in Fig. 2 is essentially the same as thatshown in Fig. 1. Each of the motors is provided with opposing iieldwindings, with a clutch brake unit and operating coil, and with aresistance operatively associated with each of the limit switches. Tosimplify the illustration. these elements are not described in detail.

The invention system may be designed to Upon a substantial difference inthe current I requirement or the first pair of motors with respect tothe second, relay will operate to disengage its armature 42 through itscontact 4| cutting all of the motors from the source of power 35.Similarly, if any one motor reaches its limit or operation before theother motors, its associated limit switch will insert a correspondingresistance in the circuit connected to the differ- I ential relay 40,thus permitting the other motors to keep on operating, until theirlimiting positions are reached. As in Fig. 1, upon operation of relay40, coil 44 maintains armature 42 diaengaged from contact 4! until suchtime as its holding circuit is broken by the push button 43.

Fig. 3 illustrates an arrangement for preventing disparity between eachmotor of a pair of motors or of two pairs of motors. For simplification,that portion of the motor and synchronizing circuits above the dash line80-60 of Fig. 2 has been omitted in Fig. 3, but is understood to besimilar to the showing thereof in Fig. 2. Each of the motors is providedwith limit switches 8i and 62. Each pair of limit switches is connectedto the opposite terminals of a, reversing switch 83, 64, 65 or 86. Thereversing switches are ganged together for operation as a unit as lows.

indicated at 61. In this instance, an additional pair or diflerentialrelays l8 and 15 are provided. Rela Ill prevents disparity between theleft-hand pair of motors l0, Ill"; and relay 15, between the right-handpair of motors 15', ii". Relay 40, in this instance, prevents disparityof the left-hand pair of motors l0, M" with respect to the righthandpair of motors I5, 15''. Relay HI includes opposing coils H and 12connected in series with reversing switches 63 and 64, respectively. Theopposite terminals of coils II and 12 are connected to contact I3normally engaged by armature 14. A holding coil I6 is provided havingone terminal connected through junction point 11 to contact 13 and theother terminal to a Junction point 18. Relay I5 is provided withopposing coils 8i and 82 connected in series with reversing switches 65and 68, respectively. The opposite terminals of coils II and 82 areconnected in parallel to contact 83 normally engaged by armature 84. Aholding coil 85 is provided havin one contact connected through junctiopoint 81 to contact 83 and its opposite terminal connected to junctionpoint 18. The junction point 18 is connected in series with holding coil44 of relay 40. As in the systems of Figs. 1 and 2, holding coil 44 hasone terminal connected to junction point 45 and the other terminalconnected to junction point 46 and is normally shunted by armature 42and contact H.

The operation of the system of Fig. 3 is as fol- If there is asubstantial disparity in the current requirements of motors Ill and I0",relay Ill will be energized in the same manner as previously described,to attract its armature 14. Armature 14 will be held open by coil I6through a holding circuit including junction point 11 and 18. Theresulting current unbalance created in relay 4!! will cause opening ofits armature 42 which is held open by holding coil 44. Opening ofarmature 42 will cause all of the motors l0, l0", l5 and I5 to bedisconnected from the source or power 25. Restoration of the system tonormal is effected by depression of push-button 43 breaking the holdingcircuit for coils 44 and 16. A similar operation, through relay 15,takes place upon a, disparity between motors l5 and Should either motorreach its limit of operation before any of the other motors, its limitswitch will open and, as in the systems of Figs. 1 and 2, a resistance,equivalent to the motor running resistance, will be inserted in serieswith the associated coil ii of relay in or 15. Accordingly, no operationof the relays will take place. That motor will stop and the other motorswill continue to operate. Ii thereafter another motor reaches its limitof operation and opens its limit switch,

an equivalent resistance is again inserted in series control mechanism,"now Patent No. 2,355,381,

issued August 8, 1944, which is assigned to the same assignee as thiscase. As in Fig. 3, the portion of the motors illustrated above the line60-450 of Fig. 2 has been omitted to simplify the drawing, but isunderstood to be similar to the showing thereof in Fig. 2. Each of the.motors is adapted connected in parallel with each other and in serieswith opposing coil is of relay II.

The control oi the motors l0, II", II and II" in this instance is by aposition preselection mechanism III of the type described and claimed inmy" said Patent No. 2,355,381. Mechanism H0 includes a pair of switches01, ll each having onetermlnal connected to one side of source throughJunction point Ill. The opposite terminal of switch I! isconnected toJunction point ill. Junction point I02 is connected to conductor I whichconnects coils I! of the interlocking relays in parallel. 'The oppositeterminal oi switch ll is connected to a Junction point Ill, and thisjunction point is connected to a conductor ill, which connects coils lloi the interlocking relays in parallel. Hence, depending upon which 01the switches ll or II is closed, coils Si or 92 of the interlockingrelays will be energized. Correspondingly, armatures It or ll of therelays will be operated to energize the motors ior operation in onedirection or the other.

Switches 91 and SI are operated by means of scam llllwhichisarrangedtoberotatedby apreselection control knob I". As describedin. my said Patent No. 2,355,381, cam I01 is adapted to be moved back toa neutral position by a reversible series-type direct current motor IIIin synchronous operation with the actuator motors. Motor i ll includesan armature H2 connected in series with a coil III which may be theenergizing winding of an electromagnetic clutch and brake unit of thetype described and claimed in my said Patent 2.287.114. Coil III isconnected in series with field windings I II and III which control thedirection of operation 01 motor Ill. windings H4 and III are connectedrespectively to Junction points Ill and I01. Accordingly, mo-

tor III is energized through one or the other of the windings dependingp n which one of switches ll or 08 is closed. In the same manner as aremotors II, ll", 15' and I5", motor III is provided with synchronizingslip rings Ill which are connected through conductors Ill to conductorsII o! the synchronizing connections for the main motors. ly, motor IIIis maintained in substantial synchronism with motors III, ill", I! andI5".

The operation of the system of Fig. 4 is as follows. Knob llll is turnedto preselect the desired positional the members driven by the motors l0,II", II and I5". Operation of knob llll turns cam I" in a direction toclose one of the switches ll or II. This energizes motor Ill through.one oi its ileld windings ill or III, and the motors i0" through ll"through the interlocking relays ll, ll, Ill and I". All the motors,including motor Ill, operate in .lynchronism until such 'timeasmotorlli'hasmoved cam I01 backtothe neutral position shown in the drawings.At such time, switch ll or I8 is opened to open the circuit of motor Illand deenelgize the interlocking relays. Thus all motor drives arestopped at the preselected position; and the clutches disengage ing oithe members in their synchronous a 8 and brake units re-engsge, to leavethe driven members at such position. The use of self-braking action onclutch disengagement assures lockpositions, an avoids overshooting orhunting.

Should a substantial current unbalance exist between the pair oi motorsll, I0" and the pair of motors II, I I", relay I will become unbalneedand attract its armature 42 to break the circuit ior all 0! the motorsincluding motor Ill. Relay armature 4! will remain attracted throughholding coil 44 until such time as push-button 43 is depressed to resetthe system for continued operation. Thus disparity, between the motorsand drives, within desired limits of design. is avoided.

It will be noted that the anti-disparity synchronizing systems discloseddisconnect all motors of a group from the source 01' power upon asubstantial disparity in the current requirements of the motors.However, should one drive reach its limit 01' operation just before theothers, this will not ailect the operation of the other motors. Suchother motors will continue to operate, independently of the firstmotor,until they likewise reach their limits of operation. Through ajudicious selection oi the appropriate number of differential relays,any number of direct current motor drives or any number 0! groups 0!motor drives may be kept in synchronous relation with anti-disparitycontrol.

While specific embodiments of the invention have been described andshown to illustrate the application-'0! the'principles thereof, it willbe understood by those skilled in the art that the invention may beotherwise embodied without departing from such principles.

What is claimed is:

1. A control circuit for a plurality of motors comprising, incombination, a pair oi electric motors each having a terminal connectedto one terminal of a source of power; synchronizing circuit connectionsbetween said motors; a diilerential relay having a pair of opposingcoils each connected in series with one of said motors, said coils beingconnected to the opposite terminal of the source of power through a backcontact of said relay; said difierential relay being eil'ective, upon acurrent unbalance between said motors. to open said back contact anddisconnect said motors from said source of power; a limit switch inseries with each motor and effective, when open, to deenergiae itsassociated motor; and a resistance associated with each motor having avalue substantialLy equal to the running resistance of the associatedmotor; each limit switch, upon opening to deenergize its associatedmotor, being eflective to insert one 01' said resistances in series withits associated diilerential relay coil. whereby said diflerential relaywill remain energized tomaintain the other motor connected to saidsource of power until its limit switch opens.

2. A control circuit for a plurality of motors comprising, incombination, a, pair of electric motors each having a terminal connectedto one terminal of a sourceoi' power; synchronizing circuit connectionsbetween said motors; a diflerentlal relay having a pair of opposingcoils each connected in series with one or said motors, said coils beingconnected to the opposite terminal of the source of power through a backcontact or said relay; said diii'erential relay being effective, upon acurrent unbalance between said motors, to. open said back contact anddisconnect said ance associated with each motor having a valuesubstantially equal to the running resistance of the associated motor;each limit switch, upon opening to deenergize its associated motor,being effective to insert one of said resistances in series with itsassociated differential relay coil, whereby said differential relay willremain energized to maintain the other motor connected to said source ofpower until its limit switch opens; means effective upon opening of saidback contact to retain the'same open; and switch means effective toclose said back contact.

3. A control circuit for a plurality of motors comprising, incombination, a first pair of electric motors; a second pair of electricmotors; synchronizing circuit connections between all said motors; eachmotor having a terminal connected to one terminal of a source of power;a first differential relay having a pair of opposing coils eachconnected in series with one of the motors of said first pair; a seconddifferential relay having a pair .of opposing coils'each connected inseries with one of the motors of said second pair; and a thirddifferential relay having a pair of opposing coils; the opposing coilsof said first and second relays each being connected through a backcontact of their associated relay in series with one coil of said thirdrelay; the opposing coils of said third relay each being connected tothe opposite terminal of the source of power through a back contact oisaid third relay; said first differential relay being effective upon asubstantial current unbalance between the motors or said first pair toopen its back contact and disconnect said first pair of motors from thesource oi power; said, second differential relay being effective upon asubstantial current unbalance between the motors of said second pair toopen its back contact and disconnect said second pair of motors from thesource of power; and said third differential relay being effective upona substantial current unbalance between said pairs of motors to open itsback contact and disconnect all of said motors from the source of power.

4. A control circuit for a plurality of motors comprising, incombination, a pair of electric motors each having a terminal connectedto one terminal of a source of power; synchronizing circuit connectionsbetween said motors; a differential relay having a pair of opposingcoils each connectedin series with one of said motors, said coils beingconnected to the opposite terminal of the source of power through a backcontact of said relay; said differential relay being eflective,

upon a current unbalance between said motors, to open said back contactand disconnect said motors from said source of power; and a holding coilincorporated in said differential relay and having one terminalconnected to the opposite terminal of the source or power and its otherterminal connected in series with each of the opposing coils of saidrelay; said back contact normally shunting said holding coil.

5. A control circuit for a plurality of motors, comprising, incombination, a pair of reversible electric motors each having a terminalconnected to one terminal of a source of power; synchronizing circuitconnections between said motors; a reversing switch associated with eachmotor for controlling its direction of rotation; a diiferential relayhaving a pair of opposing coils each connected in series with one ofsaid reversing switches; said coils being connected to the oppositeterminal of the source of power through a back contact of said relay;said difierential relay being eflective, upon a current unbalancebetween said motors, to open said back contact and disconnect saidmotors from the source-of power; and a holding coil incorporated in saiddifferential relay and having one terminal connected to the oppositeterminal of the source of power and its other terminal connectedin'series with each of the opposing coils of said relay; said backcontact normally shunting said holding coil.

6. A- control circuit for a plurality of motors comprising, incombination, a pair of reversible electric motors each having a terminalconnected to one terminal of a source of power; synchronizing circuitconnections between said motors; interlocking relay means associatedwith each motor for controlling its direction of rotation; and adiiferential relay having a pair of opposing coils each connected inseries with one of said interlocking relay means; said coils beingconnected to the opposite terminal or the source of power through a backcontact of said relay; said diflerential relay being effective, upon acurrent unbalance between said motors, to open said back contact anddisconnect said motors from the source of power.

'7. A control circuit for a plurality of motors comprising. incombination, a pair of reversible electric motors each having a terminalconnected to one terminal of a source of power; synchronizing circuitconnections between said motors; interlocking relay means associatedwith each motor for controlling its direction of rotation; adifferential relay having a pair of opposing coils each connected inseries with one of said interlocking relay means; said coils beingconnected to the opposite terminal of the source of power through a backcontact of said relay; said diflerential relay being effective, upon acurrent unbalance between said motors, to open said back contact anddisconnect said motors from the source of power; and a holding coilincorporated in said diflerentialrelay and having one terminal connectedto the opposite terminal of the source of ,power and its other terminalconnected in series with each of the opposing coils of said relay; saidback contact normally shunting said holding coil.

8. A control circuit for a plurality of motors comprising. incombination, a pair of electric motors each having a terminal connectedto one terminal of a source of power; synchronizing circuit connectionsbetween said motors; a limit switch in series with each motor andeffective, when open, to deenergize its associated motor; a resistanceassociated with each motor having a value substantially equal to therunning resistance of the associated motor; a differential relay;

said diflerential relay having a pair of opposing coils each connectedin series with one of said motors and said coils being connected to theopposite terminal of the source of power through a back contact of saidrelay and being eflective, upon a current unbalance between said motors,to open said back contact and disconnect said motors from the source ofpower; each limit switch, upon opening to deenergize its associatedmotor, being effective to insert one of said resistances in series withits associated diiierential relay coil; and a holding coil incorporatedin said diiterential relay and having one terminal connected to theopposite terminal of the source of power and its other terminalconnected in series with each of the opposing coils of said relay; saidcomprising, in combination, a pair of reversible electric motors eachhaving a terminal connected to one terminal of a source of power;synchronizing clrcuit'connections between said motors; a limit switch inseries with each motor and effective, when open, to deenergize itsassociated motor; a resistance associated with each motor having a valuesubstantially equal to the running resistance of the asssociated motor;a reversin! switch associated with each motor for controllin itsdirection of rotation; a differential relay having a pair of opposingcoils each connected in series with one of said reversing switches; saidcoils being connected to the opposite terminal of the source of powerthrough a back contact of said relay; each limit switch. upon opening todeeners ze its associated motor; being effective to insert one of saidresistances in series with its associated differential relay coil; saiddifferential relay bein effective, upon a current unbalance between saidmotors, to open said back contact and disconnect said motors from saidsource of power.

10. A control circuit for a plurality of motors, comprisin incombination. a pair of reversible electric motors each having a terminalconnected to one terminal of a source 01' power: synchronizing circ itconnections between said motors: a limit switch in series with eachmotor and effective. when open, to deenergize its associated motor; aresistance associated with each motor havin a value substantially equalto the running resistance of the associated motor; a reversing switchassociated with each motor for controlling its direction of rotation: adifferential relay having a pair of opposing coils each connected inseries with one 01' said reversing switches; said coils being connectedto the opposite terminal of the source of power through a back contactof said relay; said differential relay being effective, upon currentunbalance between said motors, to open said back contact and disconnectsaid motors from said source of power; each limit switch, upon openingto deenergize its associated motor, being effective to insert one of"said resistances in series with its associated differential relay coil;and a holding coil incorporated in said differential relay and havingone terminal connected to the opposite terminal of the source of powerand its other terminal connected in series with each oi the opposingcoils of said relay; said back contact normally shunting said holdingcoil.

11. A control circuit fora plurality of motors, comprising, incombination, a pair of reversible electric motors each having a terminalconnected to one terminal of a source of power; synchronizing circuitconnections between said motors; a limit switch in series with eachmotor and effective, when open, to deenergize its associated motor; aresistance associated with each motor having a value substantially equalto the running resistance of the associated motor; interlocking relaymeans associated with each motor for controlling its direction ofrotation; a differential relay having a pair of opposing coils eachconnected in series with one of said interlocking relay means; saidcoils being connected to the opposite terminal of the source of powerthrough a back contact of said relay; said diiferential relay beingeffective, upon a current unbalance between said motors, to open saidback contact and dis- 12 connect said motors from said one terminal ofthe source of'power; each limit switch, upon opening to deenergize itsassociated motor, being eflective to insert one 01' said resistances inseries with its associated differential relay coil.

12. A control circuit for a plurality of motors, comprising, incombination, a pair of reversible electric motors each having a terminalconnected to one terminal of a source 01' power; synchronizing circuitconnections between said motors; a limit switch in series with eachmotor and effective, when open, to deenerglze its associated motor; aresistance associated with each motor haying a value substantially equalto the running resistance of the associated motor; interlocking relaymeans associated with each motor for controlling its direction 01'rotation; a diflerential relay having a pair of opposing coils eachconnested in series with one or said interlocking relay means; saidcoils being connected to the opposite terminal of the source of powerthrough a back contact of said relay; said differential relay beingeffective, upon a current unbalance between said motors, to open saidback contact and disconnect said motors from said one terminal of thesource of power; each limit switch,

upon opening to deenergize its associated motor.

being effective to insert one of said resistances in series with itsassociated differential relay coil; and a holding coil incorporated. insaid dinerential relay and having one terminal connected to the oppositeterminal or the source of power and its other terminal connected inseries with each of the opposing coils of said relay; said back contactnormally shunting said holding coil.

13. A control circuit for a plurality of motors comprising, incombination, a pair of reversible electric motors each having a terminalconnected to one terminal of a source of power; synchronizing circuitconnections between said motors: a remote position selecting unit incircuit with said motors; said unit comprising a pair 0! switches foreffecting rotation of said motors in opposite directions, a camoperatively associated with said switches, position selecting meansoperative to displace said cam to selectively operate said switches, anda reversible motor operative when energized to restore said cam to aneutral position, said switches controlling the energize tion anddirection of rotation of the motor of said unit; synchronizing circuitconnection between the motor of said unit and said first mentionedmotors; anda differential relay having a pair of opposing coils eachconnected in series with one of said first-mentioned motors; said coilsbeing connected to the opposite terminal of the source of power througha back contact of said relay; said diflerential relay being effective,upon a current unbalance between said first-mentioned motors to opensaid back contact and disconnect said first-mentioned motors from thesource of power.

14. A control circuit for a plurality of motors comprising, incombination, a pair of reversible electric motors each having a terminalconnected to one terminal of a source of power; synchronizing circuitconnections between said motors; interlocking relay means associatedwith each motor for controlling its direction of rotation; a remotepositioning and selecting unit in circuit with said interlocking relaymeans, said unit comprising a pair of switches each operative toenergize said interlocking relay means for effecting rotation of saidmotors in opposite directions, a cam operatively associated with saidswitchu.

position selecting means operative to displace said cm .to selectivelyoperate said switches, and a reversible motor operative when energizedto restore said cam to a neutral position, said switches controlling theenergization and direction of rotation of the motor of said unit;synchronizing circuit connections between the motor 01' said unit andsaid first-mentioned motors; a differential relay having alpair ofopposing coils each connected in series with one 01' said interlockingrelay means; said coils being connected to the opposite terminal of thesource of power through a back contact of said relay; said differentialrelay being efiective, upon a current unbalance between saidfirst-mentioned motors, to open saidback contact and disconnect saidfirst-mentioned motors from the source of power.

15. A control circuit for a plurality of motors comprising, incombination, a pair of reversible electric motors each having a terminalconnected to one terminal 01' a source of power; synchronizing circuitconnections between said motors; interlocking relay means associatedwith each motor for controlling its direction of rotation; a remotepositioning and selecting unit in circuit with said interlocking relaymeans, said unit comprising a pair of switches each operative toenergize said interlocking relay means for etiecting rotation of saidmotors in opposite dime-- 14 tions, a cam operatively associated withsaid switches, position selecting means operative to displace saidcam toselectively operate said switches, and a reversible motor operative whenenergized to restore said cam to a neutral position, said switchescontrolling the energization and direction of rotation of the motor ofsaid unit; synchronizing circuit connections between the motor of saidunit and said first-mentioned motors; a differential relay having a pairof opposing coils each connected in series with one of said interlockingrelay means; said coils being connected to the opposite terminal 01' thesource of power through a back contact of said relay; said differentialrelay being effective, upon a current unbalance between saidfirst-mentioned motors, to open said back contact and disconnect saidfirst-mentioned motors from the source of power; a holding coilincorporated in said differential relay having one terminal connected tothe opposite terminal of the source or power and its other terminalconnected in series with each oi. the opposing coils of said relay; saidback contact normally shunting said holding coil: said holding coilbeing effective upon opening of said back contact to retain the sameopen; and switch means efl'ective to deenergize said holding coil toclose said back contact.

1 WILLIAM P. LEAR.

